Amplification attacks would still be a problem even if ISPs were to de-prioritize UDP traffic (for which there is exactly 0 reason, but that's another matter).

Amplification attacks would still be a problem even if ISPs were to de-prioritize UDP traffic (for which there is exactly 0 reason, but that's another matter). They would still impact anyone talking over UDP, effectively saturating the "UDP link" even if they leave TCP unaffected. And there are some pretty important protocols people actually care about that work over UDP - DNS and RTP în particular. Finally, in an ideal internet, ISPs would not have actually cared whether they are transporting UDP or TCP or SCTP or whatever else. The fact that it's basically impossible to send an IP packet that is not one of the blessed protocols over the Internet is already a major corruption of the stack model. Further gimping UDP to make TCP the only viable option would be crazy.